The invention described herein relates to a system for handling fuel pellets for a nuclear reactor and more particularly to apparatus used for feeding fuel pellets from a grinding machine to trays used for loading pellets into nuclear fuel rods.
The process practiced in manufacturing a nuclear fuel pellet includes the steps of sintering the pellet in a furnace and thereafter subjecting it to a grinding operation to help assure that its external diameter will be sufficiently small to permit loading into a fuel tube. Upon completion of the grinding operation, the pellets are transferred to a tray which then serves as a source of fuel pellet supply for fuel tube loading purposes.
The system of transfer includes a small vibratory feeder which transfers pellets from the discharge side of the grinder to a surge conveyor which includes a power driven woven metal belt. Since the surge conveyor belt speed is faster than the transfer rate of the vibratory feeder, the pellets discharged from the feeder are automatically spaced from each other on the belt and along the belt length. It is obvious that in this known design, a pellet on the belt has the same horizontal speed as the belt surface. If the horizontal motion of the pellet is stopped by a resisting force against the end of the pellet, such as a large number of abutting pellets, the pellet will slide on the belt, i.e., pellet horizontal movement stops but the belt continues to move under it. The force against the pellet that would cause it to stop must be equal to or greater than the force developed by the pellet weight times the coefficient of friction between the woven belt and pellet surfaces. Therefore, a pellet being transported on the belt will develop a horizontal force approximately equal to its weight times the coefficient of friction between the two surfaces. A resultant force developed by a series of sliding pellets is capable of pushing an equivalent number of pellets axially over a stationary plate, and this principle has been relied on in the past to move pellets to a position on the plate which sequentially dumps rows of pellets into grooves in a tray. During the time required for the compete pellet dumping and tray removal and replacement operation to take place, the pellets on the surge belt must be stopped to prevent run-over onto the dumping plate. Since the pellets pass between two parallel motor driven belts, when the last pellet, i.e. in this case the 36th pellet, moves onto the dumping plate, a clamp becomes effective to clamp the 37th pellet and pellets behind it merely stack up behind it in a horizontal straight line. Simultaneously, the motor driving the belts is stopped thus preventing pellet movement toward the dumping section of the apparatus. The clamp releases and the motor starts after the pellets are dropped into grooves in a tray.
The primary disadvantage of this design is that the line of pellets required to fill the discharge chute with pellets leaves no space for surge capacity during the time required for the tray loader to advance a new tray into loading position.